The present invention relates to a color image processing apparatus which estimates the spectral reflectance of an object to be shot, from a color image signal.
As the color imaging apparatus becomes more popular, it becomes more important to reproduce the original color of an object exactly using an image display apparatus such as a CRT monitor or a printing device such as a printer, from a color image signal acquired by a color image signal input device (to be referred to simply as an “input device” hereinafter) such as a color scanner or a digital camera.
Further, as networks connecting a plurality of devices via wiring are widely used, the accurate transmission of the color data of a color image signal and reproduction of color are becoming more important technical factors for establishing systems for remote medical check-ups and examinations in the field of telemedicine or electronic shopping, which are expected to significantly expand from now.
In order to establish a system capable of reproducing an accurate color, it is firstly required to estimate the color data of an object accurately from a color image signal obtained from an input device.
As a method of dealing with color quantitatively, the values of XYZ color system which is represented by the following equation (1) using functions of wavelength λ, x(λ), y(λ) and z(λ), corresponding to the spectral sensitivities of human vision, which is called as color matching function as defined by Commission International de I'Eclairage (CIE), and the spectrum of an object w(λ), or a uniform color space based on the XYZ color system are widely used.
                              X          =                      K            ⁢                                          ∫                                  λ                  =                  380                                780                            ⁢                                                x                  ⁡                                      (                    λ                    )                                                  ⁢                                  w                  ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                                                    ⁢                                  ⁢                  Y          =                      K            ⁢                                                  ⁢                                          ∫                                  λ                  =                  380                                780                            ⁢                                                x                  ⁡                                      (                    λ                    )                                                  ⁢                                  w                  ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                                                    ⁢                                  ⁢        Z        =                  K          ⁢                                    ∫                              λ                =                380                            780                        ⁢                                          z                ⁡                                  (                  λ                  )                                            ⁢                              w                ⁡                                  (                  λ                  )                                            ⁢                              ⅆ                λ                                                                        (        1        )            
In the above equation, K is a constant. In order to obtain color data of an object accurately from a color image signal, it is essential to accurately estimate XYZ values of an object under illumination of observation-site from the color image signal. In the case where the illumination used for acquiring the color image signal and the illumination of observation-site are equal to each other, the problem is to estimate the XYZ values of the object under the illumination used for acquiring the color image signal.
In this case in order to accurately obtain the XYZ values of an arbitrary object under the illumination used for obtaining a color image signal, it is required that the spectral sensitivities of the input device and the color matching functions should have a linear conversion relationship.
It is well known that the color image signal of an arbitrary object can be accurately converted to XYZ values by linear conversion only if the above condition called a Luther condition is satisfied.
The conversion relationship can be obtained from the relationship between the spectral sensitivities of the input device and the color matching functions; however it can be indirectly obtained from the relation between color image signals of three or more objects having independent XYZ values, and measured XYZ values.
Usually, it is very difficult to accurately measure the spectral sensitivities of the input device, and therefore the conversion relationship from color image signals to XYZ values is usually obtained by the latter method.
In the case where the spectral sensitivities of the input device do not satisfy the Luther condition in a strict sense, accurate XYZ values cannot be obtained from a color image signal for an arbitrary object. However, with regard to the specific object, the conversion relationship can be obtained by a way of least square method from the relationship between the color image signals and XYZ values for a number of colors of objects.
The format of the color chart used as the object for obtaining color data from a color image signal acquired by the input device, is standardized by ISO IT8.7, and provided by several film makers.
In order to obtain the color data of an object under illumination of observation-site different from the illumination of shooting-site used for acquiring a color image signal with respect to an arbitrary object, the product of the spectral sensitivities of the input device and the shooting-site illumination spectrum, and the product of the color matching functions and the observation-site illumination spectrum must have the relationship of linear conversions.
Such a condition depends upon the illumination spectrum, and therefore it is not practical in general. Consequently, in order to estimate the color data of an object under illumination different from that of shooting-site, it is necessary to obtain the spectral reflectance of the object.
For an accurate estimation of the spectral reflectance of an object, the method of obtaining the spectral reflectance of an object by estimating the spectrum from a lot of multi-channel images acquired by the spectral sensitivities of narrow bands, and dividing the estimated spectrum by illumination spectrum can be proposed as disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 9-172649.
Further, as disclosed in the Journal of Imaging Science and Technology Vol. 40, No. 5, Sep./Oct. 1996, p422–p430, in the case where objects to be shot are limited to particular subjects such as human skin and the spectral reflectance is represented by a linear combination of a small number of basis functions, it becomes possible to estimate the spectral reflectance from color image signals the band number of which is equal to or more than the number of basis functions representing the spectral reflectance of the object.
In these methods, the data of the spectrum of the illumination used to acquire the color image signal is necessary in addition to the spectral sensitivities of the input device.
However, it is difficult for the operator to accurately measure the spectral sensitivities of the input device. In consideration of a difference between individual input devices, change along with time or the like, the accuracy of the data provided by the maker of the device is not always sufficient.
Further, in order to obtain the data of the spectrum of the illumination for the shooting environment, a measurement device such as a spectrophotometer is required when shooting, and therefore these method cannot be easily applied to a system which employs a conventional color image input device.